This invention pertains to back light systems for flat-panel displays and, more particularly, to a back light system that produces high intensity, collimated light for very large flat-panel displays.
Large flat-panel displays made in accordance with known active matrix (or TFT) liquid crystal display technologies are typically mounted in front of a back light module which contains an array of fluorescent lamps. FPDs of this type have been increasing in size by about 1 to 2 inches diagonal yearly. The median size in 1999 for use in desk top PCs is about 15 inches diagonal view area. A few very large displays are made in the range of 20 to 25 inches diagonal. Tiled AMLCD FPDs may be made in the range of 40 inches diagonal, as described in copending U.S. patent application Ser. no. 09/368,921, assigned to the common assignee and hereby included as reference. However, tiling, as described in U.S. Pat. No. 5,661,531 and also included as reference requires extremely intense light sources with substantially collimated lighting, masked optical stacks, and pixel apertures that have very low emitted light efficiency. Thus, lighting with unusually high intensity ranges of 50,000 to 150,000 nits is desirable with uniformity over very large FPD areas. Unique designs, and control features are necessary to achieve such high intensities at reasonable wattages for consumer or business applications. Maintaining a bright and uniform illumination of the display over its entire active area is difficult to do. The intensity required for some applications and, in particular, that required for a large, tiled, seamless flat-panel LCD display causes the lamps to produce a significant amount of heat. In addition, fluorescent lamps are designed to run most efficiently at an elevated temperature, so it is desirable to operate them at their ideal design temperature, which is usually about 50 to 60 degrees Centigrade.
Small, edge-lit back light modules used in notebook or laptop PCs do not produce sufficient brightness for a large area display, nor are they capable of illuminating a large area uniformly. Thus, it is necessary to illuminate the area with an array of fluorescent lamps. The number of lamps required depends on the size of the area to be illuminated and the display brightness specifications. A large area display requires multiple lamps to illuminate it properly.
Since most displays are designed to be wider than they are tall, it is advantageous, from a reliability and power perspective, to use horizontal lamps. This results in fewer lamps and less power, since less lamp cathodes are present. The resultant proffered designs orient lamp tubes horizontally, one above the other with predetermined preferred angular and spacing relationships for increasing reflective efficiency of the back wall of the cavity.
The present invention provides a mechanism for using an array of high output and efficient fluorescent lamps for producing maximum brightness. Additionally, the back light assembly cavity of the inventive apparatus is treated with a highly diffuse and efficient reflective surface. Also added are commercially available optics, such as Brightness Enhancing Films (BEFs) and a diffuser for maximizing the output of the BEFs, reflector, and back light geometry.
The invention also provides for a very uniform light field across the back light exit surface.
The invention further provides means for incorporating a sharp cut-off collimator, as described in U.S. Pat. No. 5,903,328, hereby incorporated by reference.
Additionally, when used with the invention described in copending U.S. patent applications, Ser. Nos. 09/407,619 and 09/406,977, both filed concurrently herewith and also hereby incorporated by reference, the apparatus of this invention provides a very uniform, high luminance back light system capable of maintaining display brightness under a wide range of environments over long periods of time. It is particularly suited for illuminating a large tiled, seamless flat-panel LCD.
In accordance with the present invention, there is provided a system for uniformly distributing luminance from a back light module for a flat-panel, liquid crystal display (LCD). Fluorescent lamps are commonly used in back light modules for LCDs due to their high efficiency. Luminance from fluorescent lamps is a function of lamp tube temperature, as is the efficacy and also lamp life. This invention provides means for achieving luminance uniformity and a high degree of collimation.
A highly efficient and diffuse reflective surface treatment is disclosed. Reflection efficiency of this invention is significantly higher than other available treatments for large areas. In particular, a constant and uniform luminance output of the back light module is obtained through appropriate selection of lamps, geometry and optical components. A preferred balance of lamps, lamp spacing, reflective light back plane, and diffuser and collimating optics are chosen to produce a high brightness back light module with very high intensity output over very large surfaces. The variations in intensity over the illuminated area are minimized using light recycling in conjunction with the collimating optics. Variations are further reduced by incorporating the invention disclosed in patent application Ser. No. 09/406,977.
This invention provides means for achieving this goal through selection of combinations of components and appropriately designed geometry. A particular application is a large, tiled, flat-panel display having visually imperceptible seams as described in the aforementioned U.S. patent application, Ser. Nos. 08/652,032, 09/368,291, and U.S. Pat. No. 5,903,328. The back light module system, with thermal enhancements such as those disclosed in Ser. No. 09/406,977 and applicable controls, such as those disclosed in Ser. No. 09/407,619 provides for an efficient, reliable, large area, high intensity light source for flat-panel displays.
Additionally, optimum geometries are determined for the purpose of maximizing light output at high efficiencies, while minimizing luminance gradients across the display. These optimum geometries are also determined for maximizing light output using BEFs and light recycling.
Finally, a precise collimator such as that disclosed in Ser. No. 09/024,481 is added which eliminates light beyond a defined angle, as required in a tiled flat-panel LCD.